Cycles, Materials Baked to Texture Lost Shading
"Lighten" mode needs brighter color in order to show something

With your setup are baking the Color component of the Diffuse pass. This means that the bake texture will show only the exact color input that's being provided to every diffuse shader in the nodes of your scene; no bump map/normals are being considered in this process, so the only way your texture could show some variations comes from the Mix factor of the Color Mix node (in your case set to Lighten blending mode)

Regardless of what color texture is coming into the factor, the output of the node will always be the light grey you see in the baked texture you are showing in your question: you are infact trying to Lighten a bright color with a darker one.



The manual is not quite clear about what the blending mode is doing:


  Like bleach makes your whites whiter. Use with a mask to lighten up a little.


I may guess is adding the second color only when it's brighter than the first one, but it's a fact that using a second color darker than the first one results in a color that is always the same of the first one.